1. Field of the Invention
The present invention relates to an optical reading and writing device for reading and writing information on an optical information recording medium.
2. Description of the Prior Art
An optical reading and writing device requires control in the track direction for writing signals on or reading signals from an optical information recording medium concentrically or spirally without contacting the medium. There have been proposed various track direction control systems, one of which is the push-pull method which uses diffracted light from the signal bit or guide groove of the optical information reading and writing device to control the track direction.
FIG. 15 is an exploded perspective view of key components of the optical reading and writing device. FIG. 16 illustrates the principle of the push-pull method, and FIG. 17 illustrates the drawback of the push-pull method which is caused by the displacement of the guide groove and an objective lens from the neutral point of the objective lens by the same distance. FIG. 18 shows the tracking error signal of FIG. 17, and FIG. 19 shows the drawback of the push-pull method which is caused by the inclination of the objective lens against an optical disk. FIG. 21 is a sectional view of a prior art optical pick-up which eliminates the drawback of the push-pull method. FIG. 22 illustrates the relationship between the displacement d of the objective lens from the neutral point of the objective lens and an objective lens position detection signal LPS. FIG. 23 illustrates the relationship between the inclination .theta. of the objective lens against the optical disk and a tilt sensor signal DTS. FIG. 24 shows the relationship between the displacement d of the objective lens in the tracking direction and a normal tracking error signal TS. FIG. 25 shows the relationship between the displacement d of the objective lens in the tracking direction and the objective lens position detection signal LPS. FIG. 26 illustrates the relationship between the displacement d of the objective lens in the tracking direction and a corrected tracking error signal C-TS.
In these figures, reference numeral 1 represents an optical reading and writing device, 2 an optical information recording medium (hereinafter referred to as "information recording medium"), 3 an optical pick-up for writing information on or reading information from the recording medium 2, 4 the base of the optical pick-up 3, 5 a slide shaft for moving the optical pick-up base 4 in the horizontal direction (shown by an arrow S), 6 a movable holder provided on top of the base 4 and having an objective lens 7, and 8 a shaft provided on top of the base 4 and extending throughout the bearing 9 of the movable holder 6. 10 represents laser light irradiated from the objective lens onto a guide groove provided spirally on the recording medium 2 and forming a converging spot 12 at the center of the guide groove 11.
13 represents a convex lens on the opposite side of and in parallel to the information recording medium 2 of the objective lens 7. 14 indicates a two-piece photo detector which has two light receiving surfaces 15 and 16, and is located on the opposite side of the convex lens 13. A differential amplifier 17 has input terminals (+) and (-) connected to these two light receiving surfaces 15 and 16 of the convex lens 13, and is responsive to an output from the light receiving surfaces 15 and 16 to generate the tracking error signal TS. 18 and 19 are diffracted light distributions which are produced when the converging spot 12 is diffracted by both edges of the guide groove 11. 20 and 21 are diffracted light distributions projected onto the surface of the two-piece photo detector 14. 22 represents a balancer having a thruhole 23 at the center thereof. A thruhole 24 is also provided on the movable holder 6 corresponding to this thruhole 23.
Reference numeral 25 indicates a focusing control coil disposed at the bottom of the movable holder 6 and provided in a magnetic circuit constituted by a focusing control yoke 26 and a focusing control magnet 27. 28 represents a light emitting diode for position detection which irradiates a light beam 29 (hereinafter referred to as "LED"). 30 indicates a two-piece photo detector for position detection which is fixed to the base 4 so that a parting line becomes perpendicular to the moving direction of the balancer 22 and which receives the light beam 31 limited by the thruhole 23. Denoted at 23 is a tilt sensor which consists of a light emitting section 33, a light receiving section 34 which is a two-piece photo detecter, and a lens section 35, and is fixed to a position over a straight line connecting the objective lens 7 and the shaft of the base 4 and is opposed to the information recording medium 2.
With reference to FIG. 15, the operation of the whole optical reading and writing device will be described. The optical reading and writing device rotates the information recording medium 2, and irradiates the laser beam 10 from the objective lens 7 to the guide groove 11 for writing information on the optical disk 2 or reading information from the optical disk 2. To change the converging spot 12 of the laser light from the objective lens in accordance with the displacement of the guide groove 11 caused by the rotation of the optical disk 2, the base 4 is moved along the slide shaft 5 (shown by an arrow S) for rough positioning of the objective lens relative to the guide groove. Thereafter, the objective lens 7 is moved in the focusing direction (shown by an arrow F) and the tracking direction (shown by an arrow T) for fine positional adjustment. Therefore, such controls as a focus servo for the control of the movement of the objective lens 7 relative to the optical disk 2 in the focusing direction and a tracking servo for the control of the movement in the tracking direction are required.
With reference to FIG. 16, the principle of the push-pull method, a typical example of the tracking servo, will be described hereafter. When the objective lens 7 is located at the center of the guide groove 11, the strengths of diffracted light distributions 20 and 21 become equal in accordance with diffracted light distributions 18 and 19, and the tracking error signal TS outputted from the differential amplifier 17 becomes zero. However, when the position of the objective lens 7 relative to the guide groove 11 is changed for such reasons as the eccentricity of the information recording medium 2, the strengths of the diffracted light distributions 18 and 19 are not equal, and accordingly, the output TS of the differential amplifier 17 becomes positive or negative. Thereby, the control of the tracking direction is performed so that the output TS becomes zero, and the objective lens 7 is displaced and moved in the direction of X shown in the figure.
With reference to FIGS. 17 to 20, the problems of the push-pull method will be described in the following. FIG. 17 illustrates displaced guide groove 11 and objective lens 7 from the neutral point of the objective lens 7 by a distance d. In this condition, although the converging spot 12 is located at the center of the guide groove 11, the diffracted light distributions 20 and 21 projected onto the surface of the two-piece photo detector 14 are not equally incident upon the two light receiving surfaces 15 and 16 with the result that the output TS of the differential amplifier does not become zero. Namely, a tracking offset occurs. Therefore, the amount of the tracking offset of TS becomes large as the displacement d increases as shown in FIG. 18, and accordingly, it is impossible to perform tracking control properly.
FIG. 19 illustrates an inclination of the information recording medium 2 at an angle of .theta. against the objective lens 7. In this condition, although the converging spot 12 is located at the center of the guide groove 11, diffracted light distributions 18 and 19 become unequal, diffracted light distributions 20 and 21 projected onto the surface of the two-piece photo detector 14 are not equally incident upon the two light receiving surfaces 15 and 16 with the result that the output TS of the differential amplifier 17 does not become zero. Namely, a tracking offset occurs. Therefore, the amount of the tracking offset of TS becomes large as the inclination .theta. increases as shown in FIG. 20, and accordingly, it is impossible to perform tracking control properly.
As described in the foregoing, the push-pull method is a simple system utilizing diffracted light, but has the drawback that it is impossible to achieve a wide movable range in the tracking direction and a highly reliable tracking servo signal because the tracking error signal is offset by the displacement of the objective lens 7 in the tracking direction or the inclination of the information recording medium 2.
Improvement made on this push-pull method to overcome the aforementioned problem is shown in Japanese Patent Nos.61-198436 and 62-73435.
With reference to FIGS. 21 to 26, improvements made on the conventional push-pull method will be described hereafter.
A light beam 29 irradiated from the LED 28 becomes a light beam 31 which has a radiation range limited by the thruhole 23 of the balancer 22 provided on the movable holder 6 and is received by the two-piece photo detector 30 for positional detection. Thereafter, the movable holder 6 turns on the shaft 8, whereby the thruhole 23 is displaced, and the light beam 31 is also displaced over the two-piece photo detector 30 for positional detection. An objective lens position detection signal LPS shown in FIG. 22 can be achieved by taking a difference between outputs of the two-piece photo detector 30 for positional detection. Since a light beam irradiated from the light emitting section of the tilt sensor 32 fixed to the base 4 is reflected by the information recording medium 2 and returns to the light receiving section 34, a tilt detection signal DTS corresponding to the inclination .theta. of the information recording medium 2 against the base 4 can be achieved as shown in FIG. 23 by taking a difference between outputs of the light receiving section 34.
The tracking error signal TS of the normal push-pull method has such a waveform as to show that a tracking offset is caused by the displacement d of the objective lens 7 as shown in FIG. 24. Therefore, it is possible to achieve a corrected tracking error signal C-TS always free from tracking offset by calculating a difference between LPS shown in FIG. 25 and TS despite the displacement of the objective lens 7 in the tracking direction shown in FIG. 26. Therefore, it is possible to achieve a wide movable range in the tracking direction and perform tracking control properly. Since the base 4 has an unshown base inclination function which is actuated so that the tilt detection signal DTS becomes zero, the inclination of the base 4 against the information recording medium 2 is kept constant even if the information recording medium 2 is inclined. Namely, it is possible to prevent the occurrence of a tracking offset because the information recording medium 2 is not inclined against the objective lens.
The conventional optical reading and writing device is structured as described in the foregoing, and requires various optical devices for correcting a tracking offset such as the light emitting diode 28, the two-piece photo detector 30 for positional detection, the light emitting section 33 and the light receiving section 34. Therefore, the structure of the conventional optical reading and writing device is complex and has such problems as: (1) complex assembly due to an increased number of FPCs and lead wires of the light detecting section which supply and receive a current for photoelectrical conversion, (2) bulky size because the tilt sensor 32 needs to be opposed to the information recording medium 2, and (3) limitation of design freedom because of the complex structure of the device, for example when a disk cartridge is used to protect the information recording medium 2.